link: new applet

coreutils grew itself a tiny simplistic alternative to ln:

	Usage: link FILE LINK

	Create hard LINK to FILE

function                                             old     new   delta
link_main                                              -      75     +75
packed_usage                                       31114   31131     +17
applet_names                                        2564    2569      +5
applet_main                                         1480    1484      +4
applet_install_loc                                   185     186      +1
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(add/remove: 1/0 grow/shrink: 4/0 up/down: 102/0)             Total: 102 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

coreutils/link.c

@@ -0,0 +1,41 @@
+/*
+ * link implementation for busybox
+ *
+ * Copyright (C) 2017 Denys Vlasenko <vda.linux@googlemail.com>
+ *
+ * Licensed under GPLv2, see file LICENSE in this source tree.
+ */
+//config:config LINK
+//config:	bool "link"
+//config:	default y
+//config:	help
+//config:	  link creates hard links between files.
+
+//applet:IF_LINK(APPLET_NOFORK(link, link, BB_DIR_BIN, BB_SUID_DROP, link))
+
+//kbuild:lib-$(CONFIG_LINK) += link.o
+
+//usage:#define link_trivial_usage
+//usage:       "FILE LINK"
+//usage:#define link_full_usage "\n\n"
+//usage:       "Create hard LINK to FILE"
+
+#include "libbb.h"
+
+/* This is a NOFORK applet. Be very careful! */
+
+int link_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
+int link_main(int argc UNUSED_PARAM, char **argv)
+{
+	opt_complementary = "=2"; /* exactly 2 params */
+	getopt32(argv, "");
+	argv += optind;
+	if (link(argv[0], argv[1]) != 0) {
+		/* shared message */
+		bb_perror_msg_and_die("can't create %slink "
+					"%s to %s", "hard",
+					argv[1], argv[0]
+		);
+	}
+	return EXIT_SUCCESS;
+}